1. Field of the Invention
The present invention relates to a derailleur. More specifically, the present invention relates to a bicycle rear derailleur which is mounted on the frame of a bicycle and which is used to selectively transfer a chain to one of a plurality of sprockets mounted on the rear wheel hub axle.
2. Background Information
Bicycling is becoming an increasingly more popular form of recreation as well as a means of transportation. Moreover, bicycling has become a very popular competitive sport for both amateurs and professionals. Whether the bicycle is used for recreation, transportation or competition, the bicycle industry is constantly improving the various components of the bicycle.
Derailleurs constitute externally mounted gearshift devices that are mounted on bicycles, and particularly on sports type road racers and mountain bikes. Rear derailleurs typically have a mounting member, a link mechanism (one example of a moving mechanism), a movable member and a chain guide. The mounting member is mounted on the frame. The link mechanism is mounted between the mounting member and the movable member. The movable member can move relative to the mounting member via the link mechanism. The chain guide is mounted on the movable member so that this chain guide is free to swing. The chain guide moves the chain in order to transfer the chain to one of the rear sprockets. The chain guide has an outside plate member, an inside plate member, a guide pulley and a tension pulley. The outside plate member is mounted on the movable member so that this outside plate member is free to swing. The inside plate member is spaced from the outside plate member to form a chain receiving space between the two plate members. The guide pulley is rotatably mounted between the two plate members at the first ends of the two plate members and can engage with the chain. The tension pulley first ends of the two plate members and can engage with the chain.
In a conventional rear derailleur, a chain slip prevention part which prevents the chain from slipping off of the guide pulley is formed by bending the inside plate member toward the outside plate member between the supporting parts of both pulleys on the inside plate member (for example, see Japanese Laid-Open Patent Application No. 5-69878). This conventional chain slip prevention part is formed with a relatively narrow width. The reason for this is that inherently there is no need for a large width in order to prevent slipping of the chain. Furthermore, the end part of the chain slip prevention part that is in close proximity to the guide pulley is disposed on the tension pulley side of a straight line which is perpendicular to a straight line that connects both of the pulleys, and which is tangential to an imaginary circle that connects the tips of the teeth of the guide pulley. In other words, such a conventional chain slip prevention part is disposed so that this part is at a considerable distance from the guide pulley.
Furthermore, the inside plate member of a conventional chain guide has a guide pulley supporting part the supports the guide pulley, with the guide pulley supporting part having a circular arc shaped outer peripheral edge with a diameter that is approximately 50 to 60 percent of that of an imaginary circle that connects the tips of the teeth of the guide pulley. Thus, the guide pulley supporting part is configured so that the chain will be easily moved out of alignment during gear shifting. The outer peripheral edge of the inside plate member has a rectilinear bent part that extends rectilinearly from this circular arcuate part and that further extends from this location to the chain slip prevention part.
In the abovementioned conventional structure, when, for example, the chain is carried on the sprocket having the largest diameter in the forward-rearward direction, and the tension acting on the chain is increased. Instances may exist in which the chain guide is oriented so as to swing forward by a considerable amount in the counterclockwise direction as seen from the outside. When the chain guide swings in this manner, since the spacing of the sprocket teeth is approximately 10 mm, both ends of the chain slip prevention part disposed on the inside plate member may be forced into and become caught in the spaces between the teeth of large-diameter sprockets. When a sprocket rotates in the forward direction in a state in which the chain slip prevention part is caught between the teeth, the inside plate member may be forcibly moved by the sprocket, and there is a danger that the rear derailleur will be broken.
Meanwhile, since the chain slip prevention part disposed on a conventional inside plate member is at a considerable distance from the guide pulley in the positional relationship described above, there also may be instances under special conditions in which the chain is unable to return once the chain has left the guide pulley. Specifically, when an upshifting operation is performed in a state in which the crank is not rotating, and the chain guide moves to a sprocket on the small diameter side, the chain may slip from the tips of the teeth of the guide pulley under conditions in which the crank is rotated in reverse. The chain that has slipped from the guide pulley returns to the space between the inside and outside plates at the end part of the chain slip prevention part; however, since the end part of the chain slip prevention part is at a considerable distance from the guide pulley, the portion of the chain that has slipped from the guide pulley is positioned in a location that is at a considerable distance from the tooth parts of the guide pulley. As a result, even if the crank is rotated in the forward direction, the chain may not return to the guide pulley.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved bicycle rear derailleur. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.